OFDM modulation is well known from the prior art and is used in many telecommunications systems, such as DVB-T, ADSL, Wi-Fi (IEEE 802 a/g), WiMax (IEEE 802.16). It has the advantage of good spectral responsivity and good protection from selective frequency fading.
It will be recalled that in an OFDM system, the information symbols to be transmitted are grouped by blocks of N symbols, where N is generally a power of 2, the OFDM symbols being obtained by performing an IFFT (Inverse Fast Fourier Transform) on said blocks of information symbols. Generally, a cyclic prefix is added to the beginning of each OFDM signal to absorb the inter-symbol interference (ISI) and facilitate equalization upon reception. The OFDM signal formed by these OFDM symbols may possibly be translated in frequency.
Generally, the signal transmitted by an OFDM system can be represented in baseband by:
                                          s            a                    ⁡                      (            t            )                          =                                            E                        N                    ⁢                                    ∑              k                        ⁢                                          g                ⁡                                  (                                      t                    -                                                                  k                        ⁡                                                  (                                                      N                            +                            D                                                    )                                                                    ⁢                                              T                        c                                                                              )                                            ·                                                ∑                                      n                    =                    0                                                        N                    -                    1                                                  ⁢                                                      a                                          k                      ,                      n                                                        ⁢                                      ⅇ                                                                  -                        2                                            ⁢                      ⅈ                      ⁢                                                                                          ⁢                      π                      ⁢                                                                                          ⁢                                              n                                                  NT                          c                                                                    ⁢                                              (                                                  t                          -                                                      DT                            c                                                    -                                                                                    k                              ⁡                                                              (                                                                  N                                  +                                  D                                                                )                                                                                      ⁢                                                          T                              c                                                                                                      )                                                                                                                                                    (        1        )            where E is the power of the signal, N is the number of carrier waves of the OFDM multiplex, ak,n are the information symbols relative to the block k, belonging to a modulation alphabet M-ary, typically BPSK, QPSK or QAM, 1/Tc is the throughput of the information symbols where Tc is the “chip” time, D is the size of the cyclic prefix expressed in number of chips, g(t) is a forming impulsion for the OFDM signals having a temporal support [0,(N+D)Tc] intended to apodize the spectrum of the signal.
We have diagrammatically illustrated an OFDM signal in FIG. 1. It is made up of a sequence of OFDM symbols, each symbol having a total duration (N+D)Tc including a useful duration NTc and a guard interval of duration Tprefix=DTc, in which the cyclic prefix is located. It will be recalled that, traditionally, the cyclic prefix is a copy of the end of the OFDM symbol inside the guard interval. In certain OFDM systems, the cyclic prefixes are simply omitted, in other words the useful portions of the symbols are separated by “empty” guard intervals. This transmission technique also makes it possible to eliminate inter-symbol interference, but does not make equalization of the signal easy.
After propagation in the transmission channel, the OFDM signal received by the receiver can be expressed by:y(t)=hsa(t)+b(t)  (2)where hsa is the convolution between the OFDM signal transmitted, sa(t) is the impulse response from the transmission channel h(t), and b(t) is a random function describing the noise. It will be assumed that the length of the impulse response is shorter than the length of the guard interval, so that it will be possible to neglect the inter-symbol interference (ISI).
FIG. 2 diagrammatically illustrates the structure of an OFDM receiver.
After possibly baseband demodulation, the received signal is sampled at 210 at the chip frequency, then the samples are subjected to a serial/parallel conversion at 220 to form blocks of N+D samples. The first D samples corresponding to the guard interval are rejected and the block of the N remaining samples corresponding to the useful portion of the OFDM symbol is subjected to a FFT at 230. The demodulated symbols obtained are then subjected to a serial conversion at 240.
Eventually, assuming that the receiver is well synchronized in time and frequency, the demodulated symbols can be expressed by:âk,n=hnak,n+bk,n  (3)where hn is a complex coefficient that depends on the impulse response of the transmission channel, and bk,n is a random variable representing a noise sample.
The proper operation of this receiver requires precise synchronization in time and frequency. Indeed, it is understood that a poor time synchronization will cause gradual temporal slipping of the truncation window, and a poor frequency synchronization will cause a phase rotation of the samples, which can be represented by a multiplicative factor e2iπΔfnTc where Δf is the frequency offset between the demodulation frequency of the receiver and the carrier frequency of the OFDM multiplex.
The temporal and frequency synchronization of the receiver is generally done owing to the acquisition of a learning sequence.
The operation of this detector assumes, of course, that the parameters of the transmitted OFDM signal are known (in other words, the parameters of the OFDM symbols). “Parameters of the OFDM signal” refers here to the number N of sub-carriers, the useful duration NTc of a symbol or, equivalently, the frequency spacing 1/NTc between sub-carriers, the duration of the guard interval DTc and/or the repetition period (N+D)Tc of said symbols.
Quite often, the receiver does not know a priori the parameters of the OFDM signal and it is necessary to estimate them blindly, before any demodulation.
Several methods have been proposed to that end. They exploit the presence of the cyclic prefix in the OFDM signal and the resulting cyclostationarity properties. The estimators for the parameters are based on the auto-correlation function of the OFDM signal. An example of such an estimating method can be found in the article by P. Liu et al. entitled “A blind time-parameters estimation scheme for OFDM in multi-path channel”, published in Proc. 2005 Int'l Conference on Information, Communications and Signal Processing, vol. 1, pp. 242-247, 23-26 Sep. 2005.
These estimating methods, however, have the drawback of requiring the acquisition of a large number of OFDM symbols to calculate the auto-correlation function. Moreover, these methods do not work in the case, mentioned above, where the OFDM signal does not have cyclic prefixes. They do not work well or at all when the ratio between the prefix duration and the OFDM symbol duration, D/(D+N), is low. Indeed, in this case, the secondary peak of the auto-correlation function, due to the cyclostationarity of the signal, becomes blurred or even disappears in the noise. It is then impossible to precisely determine the gap between the main peak and the secondary peak that makes it possible to estimate the parameter NTc.
The aim of the present invention is therefore to propose a method for the blind estimation of parameters of an OFDM signal that does not have the aforementioned drawbacks.
A secondary aim of the present invention is to allow temporal and frequency synchronization of the OFDM receiver that is fast and does not require a learning sequence.